Purification method for alkylboranes



Sttes 3,013,084 Patented Dec. 12, 1961 3,013,084 PURIFICATION METHOD FOR ALKYLBORANE dairies W. Shepherd, Mars, Pa., assignor to Gallery Chemical Company, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed Aug. 12, 1953, Ser. No. 756,055 8 Claims. (Cl. 260-606.5)

This invention relates to the purification of alkyl pentaboranes and more particularly to a purification method whereby the spontaneous ignition temperature of these compounds is increased.

Alkyl pentaboranes are relatively new compounds of boron in which one or more of the hydrogen atoms of pentaborane, B H are replaced with alkyl groups. They may be produced, for example, by the reaction of an alkyl halide or an alkene with pentaborane in the presence of an aluminum halide catalyst, as disclosed in the copending, coassigned application of Wunz and Stang, Serial No. 484,586, filed January 27, 1955, and now abandoned. Many of these compounds as they are obtained from such processes and purified by conventional techniques have a spontaneous ignition temperature, i.e., the temperature at which they ignite spontaneously in air, below ordinary ambient temperature. For these reasons handling of materials, particularly in large volumes, is hazardous and must be attended by extraordinary safety precautions.

One object of this invention, therefore, is to provide a method for increasing the spontaneous ignition temperature of alkyl pentaboranes.

Another object is to provide a method for removing certain impurities from alkyl pentaboranes.

Other objects will become apparent from the following specification and claims.

This invention is based upon the discovery that alkyl pentaboranes which are purified by conventional methods contain certain trace impurities which may be removed by treatment with an active metal or metal oxide, and that such treatment raises the spontaneous ignition temperature of the alkyl pentaborane.

The spontaneous ignition temperature is the minimum temperature of which a material will ignite spontaneously in air. This temperature may be determined experimentally by a static heated crucible method, such as that described by Setchkin, J. Res. Nat. Bur. Standards, 53, 149 (1954). The spontaneous ignition temperatures mentioned herein were measured by this method.

The alkyl pentaboranes are ordinarily purified by conventional methods such as fractional distillation. The product thus obtained is relatively pure and may be used in any desired manner. For example, alkyl pentaboranes may be used as high-energy fuels in rocket engines, where they are combusted with conventional oxidizers such as liquid oxygen. It has been found, however, that there remain in the product so purified certain impurities in small or trace amounts. The nature of the impurities present depends upon the starting materials used in preparing the alkylpentaborane and the method of preparation. In general, these impurities are acidic or easily oxidized materials. For example, ethylpentaborane,

C H B H prepared from pentaborane and ethyl bromide in the presonce of an aluminum chloride catalyst, has been found to contain small amounts of hydrogen bromide, hydrogen chloride, ethyl bromide, pentaborane and ethane. While the presence of these impurities in such quantities is not detrimental to ordinary uses of the alkyl pentaborane, their presence causes the alkyl pentaborane to exhibit a relatively low spontaneous ignition temperature thus increasing the hazards of handling and making it necessary to use complex and cumbersome storage apparatus. Treatment of the alkyl pentaborane with an active metal or an oxide of an active metal substantially increases the spontaneous ignition temperature, by removing or decreasing the amount of these acidic and easily oxidized minor impurities.

The active metals and oxides which may be used in such treatment include iron, magnesium, calcium and aluminum and the oxides of these metals. Of the metals, magnesium appears to be the most effective, but the oxides in general seem to be more eflective than the metals. The best results, i.e., greatest increase in spontaneous ignition temperature in a short time of-treatment, were obtained using ferric oxide. For example, when 11.2 grams of ethyl pentaborane having a spontaneous ignition temperature below 25 C. were heated to 50 C. in admixture with 2.5 grams of ferric oxide for one-half hour, the ethyl pentaborane recovered by distillation had a spontaneous ignition temperature of 108 C. Alumina raised the spontaneous ignition temperature from below 25 C. to C., but required treatment for several hours at 25 C. to do so. Other metals and oxides, such as magnesium and calcium oxide, were generally somewhat less eifective, raising the spontaneous ignition temperature to about 40 C. under similar conditions.

The conditions for the treatment are not critical. For example, it is not necessary to use elevated temperatures, although moderate heating, e.g., 50 C., may be used if desired in order to minimize the time of treatment. Similarly, the amount of metal or oxide used may be varied; as little at 0.05 gram per milliliter of alkyl pentaborane has been used with good results.

The purification method described herein is particularly useful in the treatment of the lower-alkyl penta boranes, because these compounds when purified by conventional methods have a spontaneous ignition temperature at or about the range which may be encountered as the ambient temperature under various climatic conditions. Therefore, even a relatively small increase in the spontaneous ignition temperature is desirable since the danger of fire from accidental spilling and other handling is thereby decreased. For example, ethyl pentaborane is ordinarily spontaneously inflammable at temperatures below 25 C., the ordinary room temperature, so that this method is especially adapted for treatment of this compound.

According to the provisions of the patent statutes, I have explained the principle and mode of practicing my invention, have described what I now consider to be its best embodiments. However, I desire to have it understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

I claim:

1. In a method of purifying an alkyl pentaborane the step which comprises contacting said alkyl pentaborane with a member selected from the group consisting of iron, magnesium, calcium, aluminum, and their oxides.

2. The step according to claim 1 in which said alkyl pentaborane is a lower alkyl pentaborane.

3. The step according to claim 1 in which said alkyl pentaborane is ethyl pentaborane.

4. The step according to claim 1 in which said member is ferric oxide.

5. The step according to claim 1 in which said mem- 1 her is alumina.

6. The step according to claim 1 in which said member is magnesium.

7. The step according to claim 1 in which said member is calcium oxide.

8. A method of increasing the spontaneous ignition temperature of ethyl pentaborane which comprises contacting said ethyl pentaborane with ferric oxide.

No references cited. 

1. IN A METHOD OF PURIFYING AN ALKYL PENTABORANE THE STEP WHICH COMPRISES CONTACTING SAID ALKYL PENTABORANE WITH A MEMBER SELECTED FROM THE GROUP CONSISTING OF IRON, MAGNESIUM, CALCIUM, ALUMINUM, AND THEIR OXIDES. 